We have developed an in vivo selection system in which phage capable of selective homing to different tissues are recovered from a phage display peptide library following intravenous administration. Using this strategy, we have isolated several peptides that home to normal organs or tumors. We have shown that each of those peptides bind to different receptors that are selectively expressed on the vasculature of the target tissue. The tumor-homing peptides target receptors that are up regulated in tumor angiogenic vasculature. Marked improvement in the efficacy and toxicity profiles of peptide-guided cytotoxic drugs relative to the parental free drugs were observed in tumor-bearing mice. We now hypothesize that the genetic progression of malignant tumor cells is paralleled by epigenetic changes within non-malignant cells in tumor blood vessels. Here we propose to study the molecular heterogeneity of angiogenic vasculature during tumor progression. Based on these studies, we will test whether there are stage-specific efficacy differences among anti-angiogenic drugs. In this proposal, we will evaluate the expression of antigenic markers in tumor vasculature and in human tumor samples using phage and antibodies that recognize antigenic markers. We will determine whether the expression of distinct sets of markers (i) correlates with differential responses to therapy and (ii) serves as a prognostic factor. We will evaluate selective induction of endothelial cell apoptosis by proapoptotic peptides targeted to antigenic markers in tumor vasculature. We will also determine whether targeting of distinct sets of markers in tumor blood vessels increases the therapeutic index of such therapies; to establish organ- and tumor-homing peptides as tools for vascular imaging in vivo using selective circulating probes isolated by in vivo phage display and to select peptides that bind to endostatin in vitro and isolate endostatin receptor(s). We will correlate the outcome of endostatin treatment with the expression of the candidate endostatin receptor(s).